Microelectromechanical systems (MEMS) devices including such things as inertial sensors (e.g., capacitive, piezoelectric, and convective accelerometers and vibratory and tuning fork gyroscopes), microphones, pressure sensors, RF devices, and optical devices (e.g., optical switches) often include a number of structures that are released so as to be movable. Examples of released structures include microphone diaphragms, inertial sensor proof masses and shuttles, and suspended encapsulation layer(s) that cap sensor structures.
MEMS devices are typically formed on a substrate (e.g., a silicon or silicon-on-insulator wafer) using various micromachining techniques such as etching into the substrate and/or depositing/patterning various materials. Structures to be released are typically formed on top of one or more “sacrificial” layers of materials that are subsequently removed to release the structure. Typical sacrificial layers for MEMS wafer fabrication include an oxide layer. The oxide layer is typically removed using a wet or dry etch process. A wet etch process (e.g., buffered oxide etch) typically requires releasing holes that are carefully placed and spaced to allow for wet etch access, which can impose certain constraints on product design and processes. A dry etch process (e.g., vapor HF) generally provides more freedom in the placement and spacing of etch holes, which in turn can lead to more flexibility in the sensor design.
It has been recognized by the inventors for over a decade that tungsten has significant advantages over the silicon traditionally used as a material for manufacturing micromachined inertial instruments. In particular, if tungsten mechanical structures could be incorporated into electronics in a way similar to that taught by Sherman et al. in U.S. Pat. No. 5,417,111, then substantial improvements in the cost and accuracy could be made. The cost for a given performance could be reduced by reducing the mechanical structure size or the accuracy enhanced in a given size by exploiting the greater inertial signal from tungsten.
The reasons for this lie in the properties of tungsten metal. First, it is, like silicon, a brittle material at normal temperatures (i.e., in the sense of not being plastic). That is, it does not assume a permanent deformation when strained to an extent less than breaking. Thus it forms moving structures with stable geometry and stiffness. Second, it has a density 8.3 times that of silicon. So, a tungsten structure experiences about eight times the inertial force compared with a similar size silicon structure whereas they might be expected to get more nearly the same perturbing forces from non-inertial sources such as Brownian motion of the surrounding medium (or alternatively a tungsten structure can be approximately one-eighth the size of a silicon structure to experience the same inertial force, e.g., 4 um thickness of tungsten is approximately the same mass as 33 um thickness of silicon). Thus, one expects tungsten to yield much improved signal to noise ratio (SNR). Third, tungsten has 2.5 times greater Young's modulus than silicon. Greater structural stiffness can be obtained from a given size, making it less susceptible to perturbation. Fourth, our measurements indicate that the absolute strengths of tungsten microstructures are comparable with those of silicon. Strength is a critical parameter in designing moving structures. Fifth, unlike silicon, tungsten has electrically conductive oxides. Silicon naturally forms an insulating surface oxide which, to a varying extent over temperature, time and environmental factors, traps electrical charge. This destabilizes micromachined gyroscope and accelerometer null bias, generally the most important measure of inertial instrument accuracy. It has been found that coating active surfaces of silicon sensors with a conductor vastly improves their stability, as taught by O'Brien et al. in U.S. Pat. No. 5,205,171. Although it may, like silicon, adsorb foreign species on the surface, tungsten is free of the dominant charging effect.
Others have recognized the use of tungsten in MEMS devices would have certain advantages, particularly as a substitute for silicon. For example, U.S. Pat. No. 7,367,232 and U.S. Published Patent Application Nos. U.S. 2011/0005319 and U.S. 2011/0096623 mention tungsten as a possible material for various MEMS structures. However, the mere disclosure of tungsten as a possible material for MEMS structures does not actually disclose or enable the implementation of such tungsten MEMS structures in a usable manner. The inventors have experimented with tungsten MEMS structures (e.g., as a substitute for silicon structures) and have found that the use of traditional fabrication processes produces tungsten MEMS structures with high internal stresses such that the tungsten MEMS structures tend to warp or bend when released, resulting in devices that are unusable or of low performance.